Libellés

dimanche 4 janvier 2015

28- Water and irrigation -1- Water in agriculture

I
have not mentioned it yet, but it is a fundamental issue when we speak about
agriculture.

Indeed,
there is no life without water. Agriculture exclusively works on the living
beings, plants and animals. So there is no agriculture without water, and
therefore, no water, no food.

In
many parts of the world, rainfall is sufficient in quantity and distribution
over the year, to meet the water needs of agriculture. For example, in France,

a
country with highly developed and modern agriculture, only 7% of agricultural
fields are irrigated. In Spain, the climate is much drier, so 20% of
agricultural fields are irrigated.

This
is one of the contradictions of agriculture. More dry is the climate, so there
are less rain to restore water supplies, higher are the irrigation needs. The
less available is the water, the more it is needed.

In
fact, it's not really the way it happens. Most regions have an adequate average
annual rainfall, in theory, to ensure the needs of agriculture. The problem
lies in the distribution of rainfall and evaporation, normal physiological
process of the plant. More the evaporation increases, more the plant needs
water, to "provide the same work."

Let's
consider two European cities, Rennes, France (Brittany), with an oceanic climate
and Seville, Spain (Andalusia), with a warm Mediterranean climate.

Climate
data, of total annual rainfall are:

Rennes
between 400 and 1050 mm, 695 mm average.

Seville
between 290 and 1090 mm​​, average 530mm.

The
differences are not apparently important.

Yet
the observation compared to the average rainfall curve, shows a very different
distribution.

Rainfall
in Rennes marks a relatively flat curve, while that of Seville shows a normal
drought that lasts 4 to 5 months during the summer.

To
this phenomenon, we must add the criterion of evaporation, or more accurately
the hydric deficit, ie the difference between rainfall and evaporation.

In
Rennes, it is around 200 mm per year, while in Seville it is around 600 mm per
year. This means that in Rennes, the annual evaporation is 200 mm higher than
annual rainfall, while in Seville, it is 600 mm higher.

A
cornfield located around Rennes will not have the same requirements as the same
cornfield near Seville.

I
remind you that 1 mm of rainfall or evaporation is una quantity 1 liter of
water per m2, which is equivalent to 10,000 liters per hectare or 10 m3 / ha.

So
when I tell you that a crop requires 5,000 m3 of irrigation water per hectare
per year, it means that it needs 500 mm of additional water that is not
naturally available. It is through the irrigation water that it is possible to
offset the deficit.

Okay.
For the local agriculture, the consequences are obvious. Around Rennes, only
0.7% of the area is irrigated, while around Seville, the percentage goes to
37.7%.

In
the world, only 20% of the cultivated area is irrigated, but it provides more
than a third of global food production.

So
far, everything is logical. It rains less in Seville at Rennes, the weather is
also much warmer and evaporation is much higher.

The
problem is that in regions where rainfall is seasonal, and agriculture has a
great need for water in the dry season, it is imperative to have or to create
water reservoirs. It is necessary to be able to accumulate in dams, the water
in excess of the rainy periods, in order to dispose of it during dry periods.

This
is the case in Spain, which has developed and carried out, many years ago, a
water self-sufficiency policy.

And
even if the political and social circumstances of the construction of these
dams are highly questionable (Spain was under a dictatorship), the result is
still there, nearly 30 years after the restoration of democracy, allowing the
country to have a vibrant agriculture, and one of the most modern in the world
in terms of irrigation and plant nutrition.

It is clear
that an adaptation of structures is imperative in all regions where water
scarcity is usual. But the possible adaptations, and irrigation methods are
varied. And every situation has its own imperatives.

The
first known adaptations correspond to what is now recognized as the invention
of irrigation, and date back more than 5000 years BC in Mesopotamia.

This picture is taken from the beautiful website http://www.sienteamerica.com/posts/2472-8-paisajes-de-montanas-en-terrazas

There are
many controversies regarding the use of water in agriculture and its
consequences.

The first is
that agriculture consumes a huge amount of fresh water. That's right, according
to the FAO, over 70% of freshwater global consumption comes from agriculture.

It seems
outrageous to some? No problem. Let's stop feeding us, the problem will be
solved.

In fact all
is not so simple. Can we reduce water consumption in agriculture without
reducing yield? The answer is yes, doubtlessly.

But reducing
agricultural water has costs, especially energy cost. You have to choose
between saving water by expending more energy, or save energy by consuming more
fresh water.

The problem
is difficult and complex. I want to address these issues by trying to clarify
some things about this vast subject.

It will be a
new series simply called "water and irrigation."

I will talk
about irrigation methods, monitoring of needs of plants, water quality, reserve
management, pollution, plant physiology, etc.

The data used in this article come from Météo
France, from AEMET, from the FAO, and from the inevitable Wikipedia, along with
some personal information.